executor: fix panic in `TopN` when system variable `tidb_enable_tmp_storage_on_oom` is disabled

[xzhangxian1008]

What problem does this PR solve?

Issue Number: close #54206

Problem Summary:

What changed and how does it work?

Check List

Tests

• [ ] Unit test
• [x] Integration test
• [ ] Manual test (add detailed scripts or steps below)
• [ ] No need to test

  • [ ] I checked and no code files have been changed.

Side effects

• [ ] Performance regression: Consumes more CPU
• [ ] Performance regression: Consumes more Memory
• [ ] Breaking backward compatibility

Documentation

• [ ] Affects user behaviors
• [ ] Contains syntax changes
• [ ] Contains variable changes
• [ ] Contains experimental features
• [ ] Changes MySQL compatibility

Release note

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fix panic in TopN when system variable tidb_enable_tmp_storage_on_oom is disabled